1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly to a waveform-generating circuit device having a pulsed waveform-generating function and capable of varying a dead time or duty ratio.
2. Description of the Related Art
Referring to FIGS. 1-3, a conventional waveform-generating circuit device will be described.
FIG. 1 is a block diagram illustrating a conventional waveform-generating circuit device.
The waveform-generating circuit device (conventional example 1) shown in FIG. 1 has a function for adding a dead time. This device measures a dead time using a dead time counter 101, and generates a waveform.
FIG. 2 is a timing chart useful in explaining waveforms with respective dead-time portions generated by the waveform-generating circuit device.
Firstly, a dead time counter 101 and time counter 102 are simultaneously started. When the value of the dead time counter 101 reaches the value registered in a dead time comparison register 103, a pulse 1 rises as shown in FIG. 2. At the same time, the dead time counter 101 stops counting. Subsequently, when the value of the time counter 102 reaches the value registered in a pulse width comparison register 104, the pulse 1 drops. At the same time, the dead time counter 101 is restarted.
When the value of the dead time counter 101 reaches the value registered in the dead time comparison register 103, a pulse 2 rises as shown in FIG. 2. Upon the falling of the pulse 2, the dead time counter 101 again stops counting. Thereafter, when the value of the time counter 102 reaches the value registered in a period comparison register 103, the pulse 2 drops. At the same time, the dead time counter 101 and time counter 102 are restarted. The above processing is repeated.
As stated above, the conventional waveform-generating circuit device requires two counters. Further, where two or more pulsed waveforms having a single period are generated, the degree of freedom in designing waveforms is low, which means that a variety of pulsed waveforms cannot be generated. To obtain a high degree of freedom in waveform control, the dead time counter 101, dead time control circuit 105 and comparator circuit 106 must be dedicated to dead time setting. As a result, the circuit scale is inevitably increased to that shown in FIG. 3 (conventional example 2).
Furthermore, consideration will be given to the case where the period of a waveform is continuously varied with the duty ratio fixed at 50%. As illustrated in FIG. 11, the CPU of the conventional waveform-generating circuit device must compute the pulse width for every loop, and set a pulse width setting register 107 accordingly. Therefore, the CPU bears a heavy load and hence its response becomes low, which adversely affects the period changing operation of a high-speed time counter.